It has long been a problem to measure antenna impedance at the feed point of an antenna when the antenna has been installed or is in place and is remote from the transmitter to which is coupled. In order to be able to transmit efficiently, it is important to be able to match the output impedance of a transmitter to the input impedance of the antenna, and to do so, antenna tuners are interposed between the feed point of the antenna and the transmitter output. Antennas typically have a transmission line that connects them to the output of the transmitter, and the antenna tuners only function to sense impedance at the transmitter end of the transmission line and match that impedance to the output impedance of the transmitter.
While this couples maximum power from the transmitter to the transmitter end of the transmission line, it does not address any mismatch between transmitter output impedance and antenna input impedance. In operation of a radio station, whether it is a commercial radio station or amateur radio station, oftentimes environmental factors affect the antenna impedance. For instance, snow, rain, wind, and moisture can affect the antenna input impedance, which causes it to vary from its design parameters. Also, corrosion and wear can alter the input impedance of the antenna such that the antenna loses its efficiency. However, this loss of efficiency is not detectable at the transmitter, which is typically remote from the antenna and is coupled to the antenna by a length of transmission line. Thus, technicians at the transmitter have no way of knowing the actual condition of the antenna and cannot, for instance, be made aware of changes in the antenna input impedance. The result is that a transmitting station originally set up and optimized may not be operating in an optimal fashion, with this fact not being known to the station operator. The following details how, in the past, station engineers have remotely sensed the condition of their antennas.
Typically, station engineers have utilized antenna analyzers which couple a low-level signal, in the milliwatt range, to the antenna feed point at a predetermined frequency, with the standing wave ratio (SWR) measured by the antenna analyzer. However, due to the relatively low output of portable antenna analyzers, nearby radio stations affect the reading of the antenna analyzer. On some occasions, it is difficult to obtain reliable readings from the antenna analyzer due to high-power RF signals in the area, which tend to swamp out the relatively low antenna analyzer output signals.
Sources for these RF signals include commercial radio stations and mobile radios that serve, for instance, as taxi radios, truck radios, and bus and commercial vehicle communications. Also, high-power signals generated, for instance, by power lines, electrical transformers, local radars, and even transponders operating in the area can swamp out the antenna analyzer signals.
In order to measure antenna impedance of the antenna feed point, typically one would have to disconnect any antenna tuner utilized and, in one method, connect a 50-ohm resistor at the transmitter side of the antenna tuner connected to the antenna. Having disconnected the antenna, one then would connect the above-mentioned antenna analyzer to the antenna side of the antenna tuner and would measure the input impedance of the antenna utilizing the antenna analyzer connected to the already-tuned antenna tuner. The complex conjugate of the measured impedance at the antenna analyzer is the antenna feed point impedance.
The above procedure is quite complicated and, first and foremost, involves removing the antenna from the tuner and, second, involves using a low-power antenna analyzer, which is not the same as using actual operating conditions.
A second way to measure antenna input impedance is to interpose an antenna tuner between the antenna and the transmitter and then utilize the antenna tuner to tune the transmitter output to the impedance at the end of the coaxial cable coupled to the antenna tuner. Thereafter, one disconnects the transmitter and the antenna and installs the antenna analyzer on the transmitter side of the antenna tuner, while at the same time substituting a variable resistor, a variable capacitor, and/or a variable inductor, in series, for the removed antenna at the antenna input to the tuner. By changing all of the values of these variable elements, one then adjusts all of such values until one achieves an SWR of 1:1. When this is achieved, one determines the value of the resistance and capacitance and/or inductance to determine the feed point impedance of the antenna.
This system of measuring antenna input impedance is likewise cumbersome.
There is, therefore, a need for a simplified method to remotely measure antenna input impedance and to do so at that end of the transmission line which is coupled to the transmitter, with the measurement being made at full transmitter power so that the measurement is not swamped by local signals and is made under actual operating conditions.
It is noted that, in some instances, if impedance is not measured at full power, there can be anomalies in the measurement when measuring at a low power and then increasing power. Typically, a 1:1 SWR at low power may change when power is increased.